Kidney transplantation is the preferred and cost effective treatment for end-stage renal disease. Following transplantation, allograft rejection remains a frequent complication in spite of immunosuppressive drugs. Acute rejection (AR) in the immediate post-transplant period is the leading cause of graft loss contributing to increased morbidity and cost. Early diagnosis of AR is essential for effective treatment. Definitive diagnosis of rejection provided by biopsy, which is invasive and thus accompanies some risk. Since rejection is manifested earliest in the vascular bed, measurement of true tissue perfusion should be an early indicator of the rejection process. Perfusion MRI is proposed as a non invasive method for diagnosis of acute rejection in renal allografts. The proposed research will develop perfusion imaging of human kidneys using the MRI method of magnetic labeling arterial water. With this approach, the regional distribution of perfusion can be measured quantitative without using exogenous tracers. Perfusion MRI will be developed using the echo-planar imaging technique (EPI) as fast data acquisition enables perfusion images through the entire kidney to be acquired in a single study session. AR must be distinguished from other complications of transplantation such as acute tubular necrosis and immunosuppressive drug toxicity. This proposal will test the hypothesis that reduction in tissue perfusion as measured by MRI will be more prominent during AR than in other complications. Renal transplant recipients will be studied with perfusion MRI during the first three months following the transplantation procedure. The significance of the differences in the measured perfusion under different medical conditions, as indicated biopsy and biochemical data, will be determined. Perfusion characteristics of renal allografts measured perfusion MRI offer potential clinical application to diagnosis of AR.